In vehicle engines with direct fuel injection, e.g. diesel engines or the recently developed direct injection gasoline engines, the fuel is compressed to a certain pressure in a pressure chamber and then distributed by injection valves to the individual combustion chambers. A constant fuel pressure is maintained, depending on the conditions.
In the case of direct gasoline injection (DGI), however, the fuel pressure takes on a greater significance because it is largely responsible for the quality of fuel preparation during injection as well as the depth of fuel penetration into the combustion chamber. To utilize the full potential of an internal combustion engine with direct gasoline injection, particularly in "stratified charge mode"--in which the fuel in the combustion chamber is distributed within channels or strata--different fuel pressures are set, depending on the working point. The fuel pressure is usually varied as a function of the load applied to the internal combustion engine as well as speed. This produces a transient change from one pressure level to another pressure level.
In contrast to the spark-ignition engine, in which a relatively constant pressure is applied to the fuel, the fuel pressure in direct-injection diesel or gasoline engines is set between approximately 40 bars and 120 bars. depending on the working point. In a direct-injection gasoline engine, the working points correspond, for example to the above-mentioned stratified charge mode, a homogeneous mode, or an idle mode.
A further operating parameter is the injection geometry selected for the injection nozzles, or rather the role it plays in the penetration geometry of the fuel injected into the combustion chamber.
Because this geometry and the injection nozzle properties are assumed to be constant, the injection geometry, i.e. the detailed variation of fuel in the combustion chamber, is almost entirely dependent on the fuel pressure present at the injection nozzle.
In conventional internal combustion engines, for example the diesel injection engine described in U.S. Pat. No. 4,777,921, the fuel is supplied by a high-pressure pump to a pressure chamber, referred to as a "fuel rail". The pressurized fuel is then injected into combustion chambers from the pressure chamber by electrically driven injection valves according to the prevailing operating conditions in the internal combustion engine. The high-pressure pump is also driven by a control unit according to the prevailing operating conditions.
Due to cost and space constraints, the high-pressure pump is now becoming ever more compact. In contrast to this trend, the pressure chambers have a tendency to increase in capacity. since this inhibits, in particular, the formation of vapor bubbles in the pressurized fuel during hot operation or hot start. The main reason for this in particular is that a large-volume pressure chamber that extends almost all the way to the cylinder heads of the combustion chambers effectively prevents bubbles from forming in the pressure chamber area close to the cylinder head as a result of fuel convection. This increasingly prolongs the pressure buildup times needed in the pressure chamber due to necessary changes in fuel pressure. As a result, erroneous deviations from the desired pressure occur temporarily.
In the above-mentioned internal combustion engines with direct gasoline injection, the working points are subject to constant changes, which means that these engines are continuously within the transient range. Due to nitrogen oxide conversion, for example, the engine is constantly switching back and forth between homogeneous and stratified charge mode. In particular, this changeover is independent of the specific driver request. To this are added changes in the working point that are caused by new driver requests.